I lost my "sent" emails from my Mail 2.1

Hi everybodry,
I need your help. Yesterday I made some changes in my pop3 email account (I changed the outoing server) and after that all the messages that I had in my Sent folder dissapeared. I had more than 8 months of sent files there!!!
I looked at the library/mail/sentmessages.mbox but I just found a few of them.

I too just discovered that my Apple Mail v4.6 (1085) "Sent" folder only lists the mssgs I sent in the past 4 days; ALL previous sent messages - back for several years!! - seemingly are lost. I did nothing intentional to the software. I just returned to my ADSL and while away struggled w ratty wifi signals. There were the usual complaints of, to paraphrase, "can't use the smtp server - chose another," and infinitely slow progress completing Mail's tasks displayed in the Activity window--probably more than once Mail was not allowed to complete them, when I had to put the Mac to sleep [also not unusual]. Other than those, I can think of nothing that might be the cause of this loss.